1. Field of the Invention
The present invention generally relates to an apparatus and method for correcting Common Phase Error (CPE) in a communication system. More particularly, the present invention relates to an apparatus and method for correcting CPE in a multi-carrier communication system.
2. Description of the Related Art
In general, future-generation communication systems are under development to provide services that enable high-rate large-data transmission/reception to Mobile stations (MSs). In this context, a multi-carrier scheme, for example, Orthogonal Frequency Division Multiplexing (OFDM) is considered promising for the high-rate large-data transmission/reception.
However, a receiver in an OFDM communication system is very sensitive to phase noise caused by insufficient isolation in a local oscillator or a signal path. The phase noise may appear in a transmitter as well as in the receiver. The phase noise is an unintended additive phase modulation component, which is resolved into CPE and Inter-Carrier Interference (ICI) after Fast Fourier Transform (FFT) in the receiver. The ICI increases the variance of a detected signal in the receiver, and the CPE rotates the entire signal of one OFDM symbol equally. Hence, CPE correction is very significant in improving reception performance.
CPE correction can be carried out in three ways. One CPE correction strategy is to improve the accuracy of a device itself such as a local oscillator. Another CPE correction strategy is to reinforce isolation in a signal path, and A third CPE correction strategy is to estimate the phase difference between a transmitted signal point and a received signal point on a signal constellation.
The first and second CPE correction strategies correct the CPE in hardware, whereas the third CPE strategy is a software-based CPE correction solution. The first and second CPE correction strategies are limited in their use because the requirement for increased device accuracy leads to a cost increase. Even at the sacrifice of cost, an amplifier becomes unstable during lamp-up/down when the amplifier turns on/off in a Time Division Duplexing (TDD) communication system. That is why the third CPE correction strategy is usually adopted.
With reference to FIG. 1, a description will be made below of a soft-decision value including a CPE output from the receiver, when the transmitter uses Quadrature Phase Shift Keying (QPSK) in the OFDM communication system.
FIG. 1 illustrates a soft-decision value including a CPE output from a receiver, when a transmitter uses QPSK in a conventional OFDM communication system.
Referring to FIG. 1, the vertical axis is an imaginary (im) axis and the horizontal axis is a real (re) axis. φi denotes the average phase difference (i.e. CPE) on a signal constellation between the soft-decision values of OFDM symbol #i at a receiver and a QPSK symbol for OFDM symbol #i transmitted by the transmitter. Here, i denotes an OFDM symbol index.
With reference to FIG. 2, the internal structure of a CPE correction apparatus of the receiver in the OFDM communication system will be described.
FIG. 2 is a block diagram of a CPE correction apparatus of a receiver in a conventional OFDM communication system.
Referring to FIG. 2, the CPE correction apparatus includes an FFT unit 211, a frequency-domain equalizer 213, a subcarrier selector 215, an argument calculator (arg(·)) 217, a hard-decision unit 219, an argument calculator (arg(·)) 221, a subtractor 223, a mean calculator (mean(·)) 225, a conjugate exponent calculator (exp{−j(·)}) 227, a multiplier 229, a detector 231, and a moving average filter 233.
Upon input of a received signal, the FFT unit 211 performs FFT on the received signal and provides data subcarrier signals, that is, traffic subcarrier signals among the FFT signals to the frequency-domain equalizer 213. The traffic subcarrier signals are the soft-decision values yn,ti of traffic subcarriers, given as in equation (1),yn,ti=(hn,ti)*rn,ti  (1)where i denotes an OFDM symbol index, n denotes a receive antenna index, hn,ti denotes the channel value of traffic subcarrier #t of OFDM symbol #i received through receive antenna #n, and r denotes a signal received on traffic subcarrier #t of OFDM symbol #i through receive antenna #n.
The frequency-domain equalizer 213 equalizes the soft-decision values yn,ti in the frequency domain in accordance with a CPE estimate {circumflex over (φ)}i received from the moving average filter 233 and outputs the equalized signals ŷn,ti to the subcarrier selector 215 and the multiplier 229. The subcarrier selector 215 selects a signal with the highest value ŷn,di from among the signals ŷn,ti. Thus, ŷn,di is the maximum frequency-domain equalized soft-decision value.
The argument calculator 217 calculates the argument (i.e. phase) ∠ŷn,di of the signal ŷn,di received from the subcarrier selector 215. The hard-decision unit 219 calculates the hard-decision value sn,di of the signal ŷn,di. The argument calculator 221 calculates the argument (i.e. phase) ∠sn,di of the hard-decision value sn,di.
The subtractor 223 subtracts the phase ∠sn,di received from the argument calculator 221 from the phase ∠ŷn,di received from the argument calculator 217. The mean calculator 225 averages the difference signal received from the subtractor 223, thereby estimating the CPE. The CPE estimate {circumflex over (φ)}i is computed by equation (2).{circumflex over (φ)}i=∠ŷn,di−∠sn,di  (2)
The conjugate exponent calculator 227 calculates the conjugate exponent exp{−j{circumflex over (φ)}i } of the CPE estimate {circumflex over (φ)}i received from the mean calculator 225. The multiplier 229 multiplies a signal received from the frequency-domain equalizer 213 by the conjugate exponent exp{−j{circumflex over (φ)}i}. That is, the multiplier 229 corrects the CPE by multiplying an OFDM symbol following the current OFDM symbol, i.e. OFDM symbol #i+1 by exp{−j{circumflex over (φ)}i}. The detector 231 finally detects the signal transmitted by the transmitter using the CPE-corrected signal.
Meanwhile, the moving average filter 233 updates its filter coefficient with the CPE estimate {circumflex over (φ)}i received from the mean calculator 225 and provides the CPE estimate {circumflex over (φ)}i to the frequency-domain equalizer 213.
As described above, since the software-based CPE correction strategy uses traffic subcarriers, it is very effective in terms of overhead reduction. However, as the CPE estimate of the current OFDM symbol is compensated for in the next OFDM symbol, an accurate CPE correction is impossible if the CPE changes fast, OFDM symbol by OFDM symbol.
With reference to FIG. 3, a description will be made of CPE-incurred change of Error Vector Magnitude (EVM) in each OFDM symbol in the OFDM communication system.
FIG. 3 is a graph illustrating CPE-incurred change of Error Vector Magnitude (EVM) in each OFDM symbol in a conventional OFDM communication system.
Referring to FIG. 3, the graph illustrates change of EVM caused by CPE in every OFDM symbol when the transmitter uses four transmit antennas, Tx.ANT #1 to Tx.ANT#4. Even though the transmitter and the receiver transmit and receive a signal without forced insertion of noise, the EVM is not infinite due to a variety of hardware factors. Especially, the EVM that changes in every OFDM symbol reflects CPE-caused performance degradation indirectly. As noted from FIG. 3, the CPE is highly probable to randomly occur on an OFDM symbol basis unlike frequency offset. When the CPE rapidly changes OFDM symbol by OFDM symbol, the software-based CPE correction strategy described with reference to FIG. 2 is not viable.
The software-based CPE correction is impossible in the case where the receiver uses a decoder without hard decision, such as a turbo code and a Low Density Parity Check (LDPC) code under active consideration for future-generation communication systems. If ever the software-based CPE correction is possible, an additional hard decision unit for performing hard decision on all possible candidate symbols is required, thereby increasing hardware complexity.
Since CPE estimation relies on traffic subcarriers, the software-based CPE correction strategy has a degraded CPE estimation performance if the traffic subcarriers are in poor channel status.